1. Technical Field of the Invention
The present invention also relates to noise reduction methods and systems for tiltrotor aircraft. The present invention additionally relates to methods and systems, which utilize Higher Harmonic Control (HHC) noise reduction techniques for tiltrotor aircraft.
2. Description of the Prior Art
Tiltrotor aircraft have pivoting motors mounted at the wing tips that permit the aircraft to take-off and land like helicopters, but also fly like propeller aircraft. When operated in helicopter mode, the rotors of these aircraft generate more thrust per unit rotor disk area than helicopter rotors, thus producing higher Blade-Vortex Interaction (BVI) noise levels. BVI is a major source of noise produced by helicopters and tiltrotor aircraft during descending flight.
As a rotor descends into its own wake, large air pressure fluctuations are generated on the rotor blade surfaces as each blade interacts with the blade tip vortices generated previously. These pressure fluctuations radiate as distinctly impulsive noise, with frequency content concentrated in the sensitive audible range. This can create a community disturbance that severely restricts operations of tiltrotor aircraft in populated areas. Humans can perceive this BVI noise as particularly objectionable. In addition, BVI noise can be detected at large distances from the vehicle.
An increasing number of airports are approaching their capacity in the number of flights that can be handled daily by existing runways. As a result, the civil aviation industry has recently been considering other means of transportation, such as tiltrotor aircraft. Tiltrotor aircraft thus have great potential to relieve airport congestion due to the ability of such aircraft to take off and land vertically while flying like an airplane during cruise, thus reducing the demand on airport runways. It is anticipated that the use of tiltrotor aircraft may significantly add additional capacity to airports and reduce delays. It is also expected that a significant reduction of door-to-door trip times for passengers may be implemented utilizing tiltrotor aircraft by circumventing ground and air congestion.
Tiltrotor aircraft may thus someday be utilized to ferry passengers directly to and from vertiports located near urban areas and mass transit. However, the noise levels generated by these aircraft during landing approach have raised concerns. The noise generated by a larger, 40-passenger tiltrotor aircraft, for example, is similar to that generated by the V-22 Osprey, a current military application of a tiltrotor aircraft. Such noise may be a potential barrier to civil market penetration. The development of low-noise tiltrotors is essential to the successful implementation of this revolutionary mode of air transportation and greatly expands the utility of tiltrotor aircraft.
Passive techniques for rotorcraft noise reduction include non-traditional blade planforms, devices mounted at the blade tips, and rotor speed reduction. These methods can impose several penalties on aircraft performance and rotor structural loads, and often require a major redesign of the rotor system. Such techniques have not yet produced benefits that justify the added complexity and cost. Attempts to develop low-noise approach flight profiles have shown some potential for noise reductions. Modifying the landing profile, however, may reduce noise only moderately and can severely restrict the operations of tiltrotor aircraft during one of the most critical phases of flight.
Passive noise reductions have not been very successful. Those that require modifications to the rotor blade design generally require extensive development costs and typically involve performance penalties over the entire flight envelope. Rotor speed reduction can be effective, but is generally detrimental to hover performance and typically requires modifications to the transmission to provide higher torque, which is expensive and adds weight.
Active control devices, such as rotor-blade flaps, tabs, or variable geometry have been proposed for rotorcraft noise reduction, but have yielded only small noise reduction levels. With this approach, a blade-mounted device may be actuated either continuously or over a limited portion of the blade""s revolution. Another active control approach generally involves pitch variance of the entire rotor blade at high frequencies utilizing either a conventional swash plate or individual blade-pitch actuators. The term xe2x80x9cHigher Harmonic Control,xe2x80x9d generally represented by the acronym HHC, refers to any of these active control methods that operate throughout the entire blade revolution as a multiple (i.e., higher harmonic) of the blade rotational frequency.
Determining the proper frequency and phase (i.e., HHC setting) of the dynamic control input is critical to the effectiveness of active control methods of noise reduction and is a major difficulty in developing an effective HHC system. Closed-loop HHC systems have been proposed to reduce helicopter vibration and blade loads. These systems, however, often require complex control algorithms and sophisticated sensors to operate effectively over the entire range of flight conditions necessary for successful implementation on a rotorcraft.
Small changes in flight condition often require significant changes in the HHC setting, requiring a very responsive and robust control system. For example, to determine the proper HHC setting, prior-art systems and techniques thereof utilize measured parameters in a closed-loop feedback system that seeks to minimize or optimize those measured parameters. The feedback system utilizes an algorithm to determine an HHC setting, which is then modified based on the response of the measured parameters. This process is iterated by a computer-control system until an optimal HHC setting is identified.
Thus, development of closed-loop HHC systems can be expensive and time consuming. In most cases, however, determining suitable feedback measurements has proven to be an additional complication and a significant challenge.
The present inventors have thus concluded that a need exists for simple, yet highly effective and safe, implementation of a noise reduction system for tiltrotor aircraft based on Higher Harmonic Control (HHC) technology. The present inventors believe that a significant noise reduction can be achieved without a closed-loop system, which is a significant and novel departure from prior art assumptions.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
In accordance with addressing the shortcomings of the prior art, it is one aspect of the present invention to provide methods and systems for reducing noise generated by aircraft.
It is an additional aspect of the present invention to provide methods and systems for reducing noise generated by tiltrotor aircraft.
It is another aspect of the present invention to provide methods and systems, including hardware and software implementations thereof, for reducing noise generated by tiltrotor aircraft during descent of the tiltrotor aircraft.
It is yet another aspect of the present invention to provide methods and systems, including hardware and software implementations thereof, for reducing noise generated by tiltrotor aircraft during a variety of different flight conditions.
It is also an aspect of the present invention to provide methods and systems, including hardware and software implementations thereof, for reducing noise generated by tiltrotor aircraft utilizing a Higher Harmonic Control (HHC) input.
It is still another aspect of the present invention to provide methods and systems for reducing noise generated by tiltrotor aircraft utilizing an open-loop HHC input configuration.
In accordance with various aspects of the invention, methods and systems are disclosed herein for reducing noise generated by rotating blades of a tiltrotor aircraft. Generally, a rotor-blade pitch angle associated with the tiltrotor aircraft can be controlled utilizing a swashplate connected to the rotating blades of the tiltrotor aircraft by pitch links. One or more Higher Harmonic Control (HHC) signals can be transmitted and input to a swashplate control actuator associated with the swashplate. Rotor-blade pitch oscillations can thereafter be produced in a rotating frame of reference associated with the rotating blades to thereby reduce noise associated with the rotating blades of the tiltrotor aircraft.
The transmission of the HHC signal to the swashplate control actuator can occur in response to user input to a user input interface, such as, for example, a pilot-controlled switch in the cockpit of the tiltrotor aircraft. Additionally, transmission of the HHC signal to the swashplate control actuator can produce rotor-blade pitch oscillations of the rotating blades at 4 cycles per revolution in a rotating frame of reference associated with the rotating blades. The phase of the rotor-blade pitch oscillations can be controlled such that the 4/rev component of the rotor-blade pitch angle is a minimum at a rotor-blade azimuth angle within a range from 60 degrees up to and including 90 degrees.
The phase associated with the rotor-blade pitch oscillation can be optimized for minimum noise at a fixed phase setting. The HHC control signal can preferably be transmitted to one or more swashplate control actuators through an open-loop configuration. Finally, the phase associated with the HHC control signal can be optimized for particular flight conditions and/or descent profiles associated with the tiltrotor aircraft.